Containers for storing radioactive materials are filled in a hot cell. Operations in a hot cell such as filling the vessel with radioactive material and joining the cover to the vessel are all carried out with apparatus that is remotely-controlled from a location outside of the cell. It is desirable to keep these operations within the hot cell simple and to a minimum because of the great expense and the technical effort involved with operations that must be conducted with remotely-controlled apparatus.
Containers for the long-term storage of radioactive materials must be mechanically stable, corrosion resistant and tightly sealed. If the vessel and cover are made of steel, the mechanical strength of the container is assured and the cover can be welded to the vessel in the hot cell by a simple welding process such as with the gas-shielded arc-welding process. However, the corrosion resistance of steel is inadequate for the purpose of long-time storage.
Also, it should be added that, in the case of the steel container, a follow-up heat treatment could be required to remove micro fissures occuring as a consequence of the welding operation. This is undesirable because the radioactive material in the container too would be heated and this could lead to radioactive gas leaking from the container.
It has already been suggested to make the container out of graphite for long-term storage since graphite has an excellent resistance to corrosion. The cover made of graphite is joined to the graphite vessel under conditions of high temperature and high pressure. However, this process of joining the cover to the vessel has to be conducted in the hot cell and such an operation involving high pressure and temperature in the hot cell is expensive and difficult. Furthermore, the mechanical strength of the graphite container is less than that of the steel container.
If the cover and vessel of a container were made of steel and each is coated with a protective layer such as graphite, ceramic or enamel, then the container would have the required mechanical strength and yet be corrosion resistant except for the weld seam laid down in the hot cell. To make the weld seam secure against corrosion could involve, for example, applying a coating of corrosive resistant material of the kind mentioned above to the weld seam. This could require the application of heat to the container which has been filled with radioactive material. The heat applied to the container would be transferred to the radioactive material which could cause radioactive gas to be generated and, if micro-fissures are present in the weld seam, the gas could seep from the closed container causing a dangerous condition to operating personnel who may later have to enter the hot cell. Thus, here too, follow-up work in the hot cell is required to make the seam resistant to corrosion and so make the container suitable for the long-term storage of radioactive material.
It would therefore be advantageous, if the container were made with steel as the base material in order to obtain the desired mechanical strength and stability and, if on the outside, the container were to carry a corrosive resistant protection layer of graphite, ceramic or enamel while at the same time being adapted to permit the cover to be joined to the vessel in a hot cell by a simple welding process without the need of a follow-up heat treatment operation or other activity involving a major engineering effort in the hot cell.